(1) Physical Channels of Long Term Evolution (LTE) System and Signal Transmission Method Using the Same
FIG. 1 is a diagram illustrating physical channels used in a 3rd Generation Project Partnership (3GPP) LTE system (Evolved Universal Terrestrial Radio Access (E-UTRA), Release 8) which is an example of a mobile communication system, and a general signal transmission method using the same.
A User Equipment (UE), which newly enters a cell upon being turned on, performs an initial cell search operation such as synchronization with a base station in step S101. At this time, the UE may receive a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the base station, synchronize with the base station, and acquire information such as a cell ID. Thereafter, the UE may receive a Physical Broadcast Channel (PBCH) from the base station and acquire broadcasting information in the cell. Meanwhile, the UE may receive a downlink Reference Signal (RS) and check a downlink channel state in the initial cell search step.
The UE which completes the initial cell search operation may receive a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) according to the PDCCH information and acquire detailed system information in step S102.
Meanwhile, the UE which does not complete the access to the base station may perform a random Access Procedure in steps 103 to S106, in order to complete the access to the base station. At this time, the UE may transmit a feature sequence through a Physical Random Access Channel (PRACH) as a preamble (S103) and receive a response message to the random access through the PDCCH and the PDSCH corresponding thereto (S104). In contention-based random access excluding handover, thereafter, a contention resolution procedure including transmission (S105) of an additional PRACH and reception (S106) of the PDCCH and the PDSCH corresponding thereto may be performed.
The UE which performs the above-described procedure may perform reception (S107) of the PDCCH/PDSCH and transmission (S108) of a Physical Uplink Shared Channel (PUSCH)/Physical Uplink Control Channel (PUCCH) as a general uplink/downlink signal transmission procedure.
(2) Slot Structure of LTE System
In a cellular Orthogonal Frequency Division Multiplexing (OFDM) wireless packet communication system, uplink/downlink data packet transmission is performed in subframe units. One subframe is defined as a predetermined time duration including a plurality of OFDM symbols.
3GPP supports a type 1 radio frame structure applicable to Frequency Division Duplex (FDD) and a type 2 radio frame structure applicable to Time Division Duplex (TDD).
FIG. 2 is a diagram showing the type 1 radio frame structure. The type 1 radio frame includes 10 subframes and one subframe includes two slots.
FIG. 3 is a diagram showing the type 2 radio frame structure. The type 2 radio frame includes two half frames and each half frame includes five subframes, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS). Among them, one subframe includes two slots. The DwPTS is used for initial cell search, synchronization or channel estimation of a terminal. The UpPTS is used for channel estimation of a base station and uplink transmission synchronization of a terminal. The GP is used to eliminate interference generated in uplink due to multi-path delay of a downlink signal between uplink and downlink. That is, one subframe includes two slots regardless of the type of the radio frame.
FIG. 4 is a diagram showing an LTE downlink slot structure. As shown in FIG. 4, a signal transmitted by each slot may be expressed by a resource grid including NRBDLNSCRB subcarriers and NsymbDL OFDM symbols. Here, NRBDL denotes the number of Resource blocks (RBs) in downlink, NSCRB denotes the number of subcarriers configuring one RB, and NsymbDL denotes the number of OFDM symbols in one downlink slot.
FIG. 5 is a diagram showing an LTE uplink slot structure. As shown in FIG. 5, a signal transmitted by each slot may be expressed by a resource grid including NRBULNSCRB subcarriers and NsymbUL OFDM symbols. Here, NRBUL denotes the number of RBs in uplink, NSCRB denotes the number of subcarriers configuring one RB, and NsymbUL denotes the number of OFDM symbols in one uplink slot.
A resource element is a resource unit defined as an index (a, b) within the uplink slot and the downlink slot and indicates one subcarrier and one OFDM symbol. Here, a denotes an index on a frequency axis and b denotes an index on a time axis.
(3) Hybrid Automatic Repeat Request (HARM)
In a mobile communication system, one base station transmits or receives data to or from a plurality of terminals in one cell or sector in a radio channel environment. In a system operated in a multi-carrier form or a form similar thereto, the base station receives packet traffic from a wired Internet network and transmits the received packet traffic to the terminals using a predetermined communication scheme. At this time, determination as to when the base station transmits data to which of the terminals using which frequency band is called downlink scheduling. In addition, the base station demodulates data received from a terminal using a predetermined communication scheme and transmits packet traffic to the wired Internet network.
Determination as to when the base station allows which of the terminals to transmit uplink data using which frequency band is called uplink scheduling. In general, a terminal having a good channel state transmits or receives data using considerable time and frequency resources.
FIG. 6 is a diagram showing time and frequency RBs in time and frequency domains. The resource of the system operated in the multi-carrier form or the form similar thereto may be largely classified into a time domain and a frequency domain. The resource may be defined by an RB, which is composed of any N subcarriers and any M subframes or a predetermined time unit. At this time, N and M may be 1.
One rectangle of FIG. 6 denotes one RB, and one RB is formed by N subcarriers on one axis and the predetermined time unit on the other axis. In downlink, the base station schedules one or more RBs to a selected terminal according to a predetermined scheduling rule and the base station transmits data using the RBs allocated to the terminal. In uplink, the base station allocates one or more RBs to a selected terminal according to a predetermined scheduling rule and the terminal transmits data in uplink using the allocated resource.
A scheme for controlling an error when a frame is lost or damaged after scheduling is performed and data is transmitted includes an Automatic Repeat Request (ARQ) scheme and a Hybrid ARQ (HARQ) scheme. Basically, in the ARQ scheme, a transmitter waits for an Acknowledgement (ACK) signal after transmitting one frame and a receiver transmits the ACK signal only when the frame is normally received without error. If an error occurs in the frame, the receiver transmits a Negative-ACK (NAK) and the frame in which the error occurs is deleted from a buffer of the receiver. The transmitter transmits a subsequent frame when receiving the ACK signal and retransmits the frame when receiving the NAK signal. Unlike the ARQ scheme, in the HARQ scheme, if the received frame cannot be demodulated, the receiver transmits the NAK signal to the transmitter, but the received frame is stored in the buffer during a predetermined period of time. Then, when the frame is retransmitted, the retransmitted frame is combined with the previously received frame so as to increase a successful reception ratio.
Recently, the HARQ scheme which is more efficient than the basic ARQ scheme is widely used. The HARQ scheme includes various schemes and may be broadly classified into a synchronous HARQ scheme and an asynchronous HARQ. In addition, the HARQ scheme may be classified into a channel-adaptive HARQ scheme and a channel-non-adaptive HARQ scheme, depending on whether the amount of resources used for retransmission is adjusted according to channel states.
In the synchronous HARQ scheme, when initial transmission fails, retransmission is performed at a timing determined by a system. That is, if it is assumed that the retransmission is performed in every fourth time unit after the initial transmission fails, since the timing when the retransmission is performed is previously determined between the base station and the terminal, the timing when the retransmission is performed does not need to be reported. When a data transmission side receives a NAK message, the frame is retransmitted in every fourth time unit until an ACK message is received.
In contrast, in the asynchronous HARQ scheme, retransmission timing is determined by new scheduling or additional signaling. The retransmission timing of the frame which has previously failed to be transmitted may be changed by various factors such as a channel state.
In the channel-non-adaptive HARQ scheme, modulation of the frame, the number of used RBs, and Adaptive Modulation and Coding (AMC), all of which are determined at the time of initial transmission, are used at the time of retransmission without change. In contrast, in the channel-adaptive HARQ scheme, the modulation of the frame, the number of used RBs, and the AMC are changed according to the channel state. For example, in the channel-non-adaptive HARQ scheme, a transmitter transmits data using six RBs in initial transmission and retransmits data using six RBs in retransmission. In contrast, in the channel-adaptive HARQ scheme, even when data is transmitted using six RBs in initial transmission, the data may be retransmitted using RBs less or greater in number than 6 according to the channel state. Based on such classification, a combination of the four HARQ schemes may be considered, but the mainly used HARQ schemes include an asynchronous channel-adaptive HARQ scheme and a synchronous channel-non-adaptive HAQR scheme.
In the asynchronous channel-adaptive HARQ scheme, the retransmission timing and the amount of resources are adaptively changed according to the channel state so as to maximize retransmission efficiency. However, since overhead is increased, this scheme is not considered in uplink.
Meanwhile, in the synchronous channel-non-adaptive HAQR scheme, since the retransmission timing and the resource allocation are determined by the system, almost no overhead occurs, but retransmission efficiency is low if this scheme is used in an environment in which the channel state is considerably changed.
In the current 3GPP LTE, the asynchronous HARQ scheme is used in downlink and the synchronous HARQ scheme is used in uplink.
FIG. 7 is a diagram illustrating resource allocation and retransmission of the asynchronous HARQ scheme. For example, in downlink, time delay occurs as shown in FIG. 7 while scheduling is performed, data is transmitted, ACK/NAK information is received from a terminal, and next data is transmitted. This time delay occurs due to channel propagation delay and time consumed for data decoding and data encoding. A transmission method using independent HARQ processes is used for gapless data transmission during a delay period. For example, if a shortest period from current data transmission to next data transmission is 7 subframes, data can be transmitted by performing 7 independent processes without gap. In the LTE, a maximum of 8 processes may be allocated if a Multiple Input Multiple Output (MIMO) scheme is not used.
(4) Multi-Carrier
The concept of multi-carrier and the concept of Component Carrier (CC) will be described. FIG. 8 is a diagram illustrating a frequency band used in a system supporting multi-carrier. In FIG. 8, multi-carrier denotes the entire frequency band used by the base station and is equal to the whole band. For example, the multi-carrier may be 100 MHz.
The CC refers to an element carrier configuring the multi-carrier. That is, a plurality of CCs configures a multi-CC through carrier aggregation. The CC includes a plurality of lower bands. At this time, if the term ‘multi-carrier’ is replaced with the term ‘whole band’. CC aggregation is also called bandwidth aggregation. As a subband, the lower band may be replaced with a partial band. In addition, the carrier aggregation can extend a bandwidth by collecting a plurality of carriers in order to increase a data rate. For example, in the existing system, one carrier is 20 MHz. However, the bandwidth can be extended to 100 MHz by collecting five carriers each having 20 MHz. The carrier aggregation includes aggregation of carriers located in different frequency bands.
At this time, if the multi-carrier is applied to the system, research into how the above-described HARQ processes are configured is necessary.